In an optical system, a transmissive optical element such as a lens or window will often exhibit slightly different indexes of refraction to respective different wavelengths of radiation. This is commonly referred to as the dispersion characteristic of the component. When different wavelengths pass through the optical component, they can be displaced relative to each other. As a result, they will typically have different optical path lengths within the component, and will ultimately focus at different locations along an optical axis. If the different wavelengths pass through multiple optical components, the effects of different optical path lengths for respective wavelengths can be cumulative.
To design a viable optical system that functions at two or more wavelengths, it is generally necessary to minimize dispersive effects. However, minimizing dispersive effects adds a significant level of complexity to the design process, particularly as the system's range of operational wavelengths increases. Not only must suitable optical materials such as glasses be selected for the various optical components, but suitable shapes, a suitable order, and suitable positions must also be determined. The materials, shapes, order and positions of the optical components interact, and thus a designer usually must spend a great deal of time balancing all of these interacting factors in order to find an acceptable combination. Although pre-existing techniques of this general type have been generally adequate for their intended purposes, they have not been satisfactory in all respects.